Identifying an indicator compound for progress monitoring during in-situ thermal treatment of coal tar and creosote

Abstract

In situ thermal treatment (ISTT) has been used to treat sites impacted by coal tar and creosote when stringent remediation objectives must be met over short timeframes. There is a need to identify an indicator compound that can be used to track progress during the treatment of these complex semi-volatile non-aqueous phase liquids (NAPLs) to complement soil sampling typically conducted once treatment is complete. This study outlines an approach to track ISTT progress and support shutdown decisions based on mass removal objectives using a series of laboratory experiments to investigate changes in semi-volatile NAPL composition during thermal treatment. Sand, water and semi-volatile NAPL were heated, and the recovery of polycyclic aromatic hydrocarbons (PAHs) was monitored by sampling and analysis of condensate. PAHs were predominantly removed between 260 °C and 455 °C, with early-stage condensate composed of higher volatility PAHs and later-stage condensate composed of lower volatility PAHs. Experimental results showed that intermediate-volatility PAHs (e.g., phenanthrene) could be used as an indicator compound to infer treatment progress with respect to both higher and lower volatility PAHs. Monitoring an indicator compound during ISTT of semi-volatile NAPL could provide higher confidence in treatment progress than conventional monitoring techniques and allow for more accurate shutdown decisions.